One idea could be a crossover with the Naruto universe. Gray could meet Naruto and they could team up to fight a common enemy. Gray's ice - magic skills would be an interesting contrast to Naruto's ninja techniques.
There are fanfictions that focus on Gray's past. For example, a story might explore his early days of training with Ur and how those experiences shaped him into the person he is in the main series. It could delve deep into his emotions during that time, like his determination to become strong after the loss he faced.
One idea could be a crossover with the Metal Gear Solid series and a cyberpunk universe. Gray Fox could be reprogrammed by a rogue AI in the new universe and have to fight against his old allies while trying to regain his true self.
A crossover with Fullmetal Alchemist could also be fascinating. Picture Edward Elric and the D Gray - Man characters collaborating to understand the mysteries of the Gate and the Innocence. Maybe they find out that there are connections between the two in terms of the source of power and the balance of the world. They could also face off against enemies that are a combination of Homunculi and Akuma.
To start writing, create a strong opening. If it's a crossover with the Fairy Tail and the DC Comics universe, you could start with Gray being transported to Gotham City. Describe the city from his perspective. Then, introduce a conflict that involves both Gray's powers and the villains or heroes of Gotham. As you write, keep in mind the personalities of both Gray and the new characters he'll interact with, and build the story from there.
One idea could be that the OC is a new mage in the guild and Gray initially misunderstands her intentions, thinking she's there to cause trouble. But as they go on missions together, he realizes she has a pure heart and they start to develop feelings. For example, they could be on a mission to save a small village from a dark wizard's curse, and the OC uses her unique magic to break the curse, impressing Gray.
Since 'lemon' fanfiction often crosses the line into adult content, we should focus on the proper characteristics of Gray in regular fanfiction. Gray is known for his stoicism, his rivalry with Natsu, and his strong sense of justice. These traits should be the focus rather than any improper aspects that might be in 'lemon' fanfiction.
One common theme is romance. Since Gray is a popular character, many fanfictions might pair the female version of him with other characters from Fairy Tail, like Natsu or Lucy, exploring different romantic relationships. Another theme could be adventure. Writers might create stories where female Gray embarks on quests or battles similar to what we see in the original series.
Well, in the world of Gray Fullbuster OC fanfiction, there are stories where the OC has a special connection to Gray's past. Maybe the OC knew Gray's family before he lost them. This could lead to a story full of revelations and emotional moments. Also, there could be a fanfiction where an OC is a time - traveler and meets Gray at different points in his life, changing the course of his story in unexpected ways.
One idea could be to have characters from 'D Gray - Man' interact with characters from another series in a post - apocalyptic setting. For example, they could team up to fight against a new type of Akuma that has emerged due to the crossover's unique circumstances.
One idea could be that the characters from 'Bleach' and 'D.Gray - Man' are brought together to fight a common enemy that is a fusion of Hollows and Akuma. For example, Ichigo and Allen could team up, with Ichigo's powerful Zangetsu and Allen's Innocence combining their powers in unique ways to defeat this new threat.